Examples of known photochromic compounds showing photochromism include diarylethene (Patent Literature 1), spiropyran (Patent Literature 2), hexaarylbiimidazole (Non Patent Literature 1), and azobenzene (Non Patent Literature 2) compounds. “Photochromism” is also called “photochromy” or “phototropism (phototropy)” and refers to a phenomenon of some types of materials in a solid or solution state changing its color by absorption of light (ultraviolet or visible light) having a specific wavelength and retrieving the original color by absorption of light (ultraviolet or visible light) having another wavelength or by cessation of the absorption of light.
Since the photochromic compounds reversibly develop the colors by absorption of light, they are being studied to be used as a light-modulating material (Patent Literature 3) or an optical recording material (Patent Literatures 4 and 5). Patent Literature 6 describes an ink containing a photochromic compound and having light stability to maintain the color density even after absorption of light for a long time.
Patent Literature 7, which is an application by the present inventors, discloses a security ink containing a photochromic compound. A security ink layer is formed with this security ink by, for example, a coating or printing process. When the security ink layer is irradiated with light, such as ultraviolet light, a security mark such as a character or drawing formed by the coating or printing process becomes visible as a result of color development of the photochromic compound and becomes invisible as a result of color quenching of the photochromic compound by cessation of the irradiation. It should be noted that Patent Literature 7 does not describe any security ink containing two or more compounds having different photochromic characteristics.